rgw: handle stripe transition when flushing final pending_data_bl

rgw: handle stripe transition when flushing final pending_data_bl

Fixes: http://tracker.ceph.com/issues/15745
When complete_writing_data() is called, if pending_data_bl is not empty
we still need to handle stripe transition correctly. If pending_data_bl
has more data that we can allow in current stripe, move to the next one.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
(cherry picked from commit b7a7d41839558b1a8786a66e27d9d80e606f61c7)

Conflicts:
	src/rgw/rgw_rados.cc (hammer write_data() takes rgw_obj reference as
	4th argument, hammer throttle_data() takes rgw_obj as 2nd argument)